Chalcedony

Chalcedony, or cryptocrystalline quartz, is a unique mineral with
a fascinating history. Many of its colorful varieties were highly esteemed
by early peoples, both for personal adornment and utilitarian articles.
Today, even though it is found in abundance throughout the word and
is one of the least expensive gem materials, it enjoys a greater popularity
than ever before. The seemingly endless array of colors and patterns
of color distribution in which chalcedony occurs presents a never ending
source of pleasure and a constant challenge, to the amateur lapidary
and "rock hound".

The origin of the world chalcedony (pronounced kal-SED-oh-nee) is
uncertain. The obvious belief is that it came from "Chalcedon" a sea
ports in Asia Minor near Byzantium, and a probable early source. The
Greek word is "chalkedon", in Latin it is "chalcedonius" The use of
the modern spelling can be traced only to the early part of the sixteenth
century.

Most of the chalcedony varieties have an intriguing historical background
and more than the usual share of legends, superstitions and strange
beliefs. Agate was one of the first gem materials known to man. According
to legend, it made the wearer agreeable and persuasive and gave him
the favor of God; too, it assured him of victory and strength.

It was supposed to cure insomnia and make the owner have pleasant
dreams. It was also said to assist the owner in acquiring riches by
making him more cautious and prudent. The Sumerians, the earliest inhabitants
of Mesopotamia, seem to have been the first to use agate and other forms
of chalcedonic quartz for seals, signet rings, beads and other articles
of jewelry. Since the Sumerian culture apparently was well developed
when the Egyptian was in its infancy, it can be seen that chalcedony
was used and cherished from earliest civilization. The Sumerians also
used agate for making ceremonial axe heads, an excellent example of
which is in the American Museum of Natural History in New York City.
An engraved inscription sets the date of the instrument at between 3000
and 2300 B.C.. In early times, agate was a favorite material for carving
into bowls, bottles and cups. An excellent example, cut from a single
piece of agate, measures twenty eight and one-half inches in diameter.
This piece was taken as loot by the Crusaders and carried to Europe;
it is now preserved in Vienna. What is often said to be the most outstanding
object fashioned from agate is a two-handled wine cup, with a capacity
of more than one pint, intricately carved on the outside with Bacchanalian
subjects. Historical records tell us that it was made for the Emperor
Nero, and that, after being owned by many persons, was give in the ninth
century by Charles the Bold to the Abbey of St. Denis, where it was
used for centuries to hold the wine at the coronation of the kings of
France. This cup is still, considered by the French to be one of the
most valuable mementos of their history. The Persians, Arabs and other
oriental peoples had great regard for the various types of agate, undoubtedly
because of its color and durability. They used it principally for finger
rings, upon which was carved a verse from the Koran (in more recent
periods), the owner's name, or some magical or symbolic figure. Such
rings were, and still are, considered powerful talismans, protecting
the owner from a wide variety of calamities. The name agate comes through
the Latin from a Greek word for the river Achates, in Sicily, where
the material was first found in quantity in the time of Theophrastus.
Mocha stone, another name for moss agate, is derived from a port on
the Red sea coast of Arabia called Mokha, once famous but now little
used.

The word chrysoprase comes from the Greek meaning "golden green".
One of the superstitions associated with this yellowish-green variety
states that if a thief sentenced to death holds a chrysoprase in his
mouth he will escape his fate. Although the legend does not describe
how this miracle was accomplished, historical records indicate that
the stone was believed to make the prisoner invisible. Another interesting
story concerns a chrysoprase that Alexander the Great wore in his girdle
during all of his battles. According to this fable, he laid aside his
girdle to bathe in the Euphrates River after his Indian campaign and
a serpent bit off the stone and dropped it into the river.

Carnelian, the original but now obsolete spelling of carnelian, was
derived from the old Latin word "cornum" meaning "cornel berry" or "cornelian
cherry". The newer spelling came into existence during the fifteenth
century, as a result of the mistaken belief that it originated from
the Latin word "carneolus", after "carnem" meaning "flesh". Legend says
that this red to brownish-red to orange-red variety had the power to
drive away evil and bring good luck. It was supposedly the stone for
weak or timid voices, giving courage to speak boldly and well. It protected
against the envious and was responsible for making sure that desires
were gratified. The popularity of carnelian among Mohammedan people
may have been due to the fact that Mohammed himself wore one as a signet
ring.

Sard, which is similar to carnelian but darker and more brownish,
comes through the Latin from the Greek word "Sardis", which was the
capital of Lydia in Asia Minor. Sardonyx, a name that describes stones
consisting of bands of sard and black and white levers, alternates with
peridot as the birthstone for the month of August. Sard was long thought
to be a protection against incantations and sorcery and was believed
to sharpen the whites, rendering the wearer fearless, victorious end
happy. The color of this stone was supposed to neutralize the malign
influence of the dark onyx, driving away the bad dreams caused by the
latter and dispelling the melancholy thoughts inspired.

Bloodstone, or heliotrope (HEEL-ee-oh-trope), the dark green chalcedony
spotted with red, alternates with aquamarine as the March birthstone.
Heliotrope comes from two Greek words meaning "sun" and "direction".
Bloodstone was once used as an amulet in the belief that it would stop
bleeding. As with many of the chalcedony varieties, bloodstone has been
used as a gemstone since earliest recorded history.

According to legend, it could cause the sun to turn red and make
thunder and lightning occur. It was supposed to give the wearer clairvoyance
and preserve his faculties and health. It meant respect to the wearer
and guarded him from deception. In addition, he was supposed to be given
what he asked for and to be believed, regardless of what he said. All
doors were open to the owner of a carved bloodstone.

Since quartz is the most abundant mineral in the earth's crust and
the minutely crystalline chalcedonic form is found in every country, it
is not surprising that there are a large number
of distinct varieties. Although hobbyist rock and mineral collectors
distinguish literally hundreds of varieties of chalcedony, those
described below are the only ones of significance to the jeweler.

Chalcedony

In general usage, the terms chalcedony and cryptocrystalline quartz
are synonymous; therefore, chalcedony may be applied correctly to
any of the numerous varieties of this large mineral family. However,
it is also sometimes used as a specific variety name to refer to the
semi-transparent to translucent white to pale grayish-blue material.

Chalcedony "Moonstone "

This variety is semitransparent white to gray (milky) chalcedony
that only vaguely resembles moonstone. It lacks the floating light
effect that is characteristic of true adularescence in genuine
moonstone. Buy
moonstone necklaces

Chrysoprase

Semitransparent to translucent light to medium yellowish-green chalcedony
is called chrysoprase. This term should be reserved for the natural
material; it should not be applied to the dyed dark green
chalcedonic quartz that is often sold as either "green onyx" or
"chrysoprase"

Carnelian

Semitransparent to translucent red, orange-red to brownish-red or
brownish-orange chalcedony is called carnelian. These colors may
also be produced by heat treating nearly colorless chalcedony. Buy
carnelian necklaces

Sard

Sard is similar to carnelian, except that its color is usually less
intense; i.e., more brownish and somewhat darker. It tends more to
the dark-reddish colors and carnelian to the lighter, predominantly
orange hues. Unfortunately, there is no distinct dividing line
between sard and carnelian.

Bloodstone or Heliotrope

Semi translucent to opaque dark-green chalcedony with red to
brownish-red spots is called either bloodstone or heliotrope. It is
often used for men's rings, either as flat tablets or in carved
forms. Buy
Bloodstone necklaces

Prase

The term prase is applied to translucent light-grayish yellow-green
chalcedony. It finds little application in the jewelry trade.

Agate

The term agate is applied correctly to chalcedony in which the color
is distributed in curved bands or layers. The banded appearance is
distinguished by a difference in either color or translucency, or
both, between adjoining layers of chalcedony. The word agate is
often used somewhat loosely with a prefix to describe material that
is not banded; e.g. landscape agate (containing inclusions
resembling a landscape), fortification agate (composed of straight,
intersecting bands), and moss agate or mocha stone (milky-white
chalcedony with green, black or brown inclusions distributed in
dendritic patterns). Because of its rather porous nature, gray and white agate is
particularly susceptible to dyes of various colors. The dye may
color the material unequally, preserving the banded appearance, or
it may penetrate porous materials so evenly that a single color
results. Buy
agate necklaces

Onyx

The only proper use of the term onyx is to describe chalcedony
composed of straight, parallel bands. It is used incorrectly when
applied to grayish chalcedony or agate that has been dyed to produce
the solid-colored material known e s "black onyx", "green onyx",
etc. These misnomers have been used for this purpose for so long and
so consistently that many jewelers do not realize that they are
incorrect and misleading. If the onyx exhibits
bands of sard colors alternating with either white or black parallel
bands, it is known as sardonyx. Cornelian onyx has alternate
parallel bands of carnelian colors with either black or white. White banding is much more common than black; in fact, natural black
chalcedony is not too frequently encountered. A cross-section of
chalcedony showing both curved bands (agate) and straight, parallel
bands (onyx).

Jasper

The name jasper is applied to most of the semi translucent to opaque
chalcedonies that occur throughout the word in a wide variety of
colors, often without a pattern of any kind. It may be red, yellow,
brown, green, grayish blue or any combination thereof. It is
frequently dyed blue and sold as "Swiss lapis" or "German lapis".

Plasma

Semi translucent to almost opaque dark-green chalcedony with white or
yellowish spots is called plasma. It is essentially bloodstone but
lacks the red spots. Both plasma and prase
are sometimes used as inexpensive jade substitutes.

Chrysocolla Chalcedony

Translucent to semi-translucent intense light-blue or blue-green
chalcedony, the color of which is caused by minutely distributed chrysocolla (a copper silicate), is one of the loveliest varieties
of chalcedony. In
its finest quality it rivals the most beautiful turquoise in
appearance. Unfortunately, however, it is quite rare.

Agatized Wood

Wood that has been replaced entirely by chalcedony resembles a
patterned jasper, and is a rather attractive material for
ornamental purposes. Almost all chalcedonic colors are represented,
including red, yellow, black and brown. It is also called
silicified or petrified wood.

Cryptocrystalline quartz is deposited at relatively low temperatures
by circulating ground waters or by magmatic waters that have dropped to
temperatures approaching those expected at the earth's surface. The
earth's crust is so rich in silica that ground waters commonly approach
saturation in silica content. As a result, chalcedony or other forms of
quartz are often deposited in cavities in the near-surface rocks. Such
waters may dissolve away pre-existing materials such as wood, bone and
shells and replace them particle by particle with chalcedony. Chalcedony
and/or crystalline quartz may fill or line cavities of any kind in
near-surface rocks to form concretions or geodes.

Sources and Recovery of chalcedony

Because chalcedony is distributed widely throughout the world, any
attempt to list and describe in detail all sources would be impractical
in a work of this nature. Listed below, however, are some of the more
notable localities.

White to pale grayish-blue chalcedony:- Siberia,
Ice-land, India and the beach and desert regions of California.

Although chalcedony has the durability to make it common in alluvial
deposits, profitable mining is usually confined to primary deposits. The
low price of most varieties precludes the practicability of sorting gem
gravels to select satisfactory pebbles. Since the bulk of the material
is heated or dyed to improve color, masses of cheap gray agate are
blasted out of extensive deposits for this purpose.

Chalcedony is fashioned in a wide variety of shapes and forms,
including beads, carved and engraved gems, pen stands, figurines,
spheres, bowls and every variation of the cabochon cut. The larger
piece, of agatized wood, agate and jasper are particularly popular for
bookends, paper weights and similar objects. The majority of agates for
commercial consumption are stained and cut at Idar-Oberstein, Germany.
Large quantities of almost all chalcedony varieties are tumbled for use
in earrings, dangle bracelets, pins and many other-jewelry articles.

Since chalcedony is one of the tougher gemstones, cutting does not
present any of the usual problems tor the lapidary. Orientation is
necessary only to obtain the desired pattern (scenes, flowers, trees,
etc., in agate), or for color or fractures. It is not overly affected by
heat, and only the usual care in dopping, grinding and sanding is
necessary. Chalcedony is one of the easiest gemstones to fashion; hence,
it is widely used in introducing the novice to the lapidary arts. It is
usually polished on a felt lap, using tin oxide or cerium oxide as the
polishing agent. A few of the jaspers that tend to undercut should be
polished with Linde A or chromic oxide on a leather lap.

Because the Idar-Oberstein district is a world famous agate cutting
center, a brief description of its activities and history is appropriate
in this assignment. Idar and Oberstein, which are twin towns in the Idar
Valley of the Nahe River in Germany's lower Saar Basin, have been noted
for their lapidary craftsmen for almost two thousand years. One of the
sights of Oberstein, which has a population of about 20,000, is an
ancient church high up on a cliff above the river that is partly carved
out of an old agate bearing lava flow. Idar, with about 12,000
inhabitants, forms that part of the twin town that lies in the
steep-walled tributary of Idar Brook. The surrounding hills consist of
old basaltic lava, in the steam cavities of which occur numerous
concentrations of agate, crystalline quartz and calcite.

Legend and history have it that the industry had its inception in
Roman times, when Caesar was pressing northward after his conquest of
Gaul. It is said that in those days a troop of Roman soldiers crossed a
mountain range and stumbled upon a small and unpretentious group of
dwellings perched on precipitous walls of rock between which ran a
rapidly flowing stream. The native presents and hunters all had
collections of beautifully marked stones that they had broken from the
rocks surroundings their village. Among these collections, the Romans,
the Romans recognized the same materials from which the artisans and
goldsmiths of the Imperial City cut and polished the precious stones
that were so prevalently worn at that time. Thus began a period of
lively activity in the little settlement, which was then known as
Hidera. The water power of Idar Brook was harnessed and great
grindstones were installed. By the year 31 A.D., Idar was an important
gem-cutting center.

Although demand diminished with the fall of the Roman Empire, the
inhabitants never ceased their activity in the handicraft that had been
introduced to them. During the twelfth century, the Counts of Oberstein,
among other German lords who owned the lands and the quarries,
controlled the important business of supplying gems to adorn the armor
and swords of the knights and the attire of the castle maidens. The
fifteenth century saw a well-established lapidary industry that was
devoted entirely to cutting the material mined from local deposits. A
cutters guild was organized in 1609 that included three specific working
groups; the stone pickers, the grinders and the drillers. Soon the fame
of Idar-Oberstein and its superior craftsmen began to spread, and many
foreign buyers came to the Idar Valley. In the early nineteenth century, when the supply of
rough material became almost nonexistent, the industry managed to
continue on a reduced scale by using Swiss rock crystal and smoky
quartz, Saxon and Bohemian amethyst, and Indian carnelian, moss agate
and jasper. In 1827 the Uruguay-Brazil agate fields were discovered by
emigrants from the Idar district, and the importation of this material
revived the languishing industry.

Today, the cutting works are located not only in the twin towns but
in the surrounding villages and farms as well. The cutting of agate
offers a supplementary source of income to farmers, woodcutters, and
others during winter months and other periods of slack time. In past
years, the numerous mountain streams furnished waterpower for the small
mills along their banks. Electrically driven machinery has replaced the
more picturesque waterwheels. The old fashioned manner of agate cutting
so frequently illustrated in descriptions of Idar-Oberstein, which
required that the cutter lie prone before a large sands tone-wheel, is
now obsolete. Although Idar was fortunate in being saved from
destruction during World War II, the industry suffered greatly as a
result of lack of material and import and export difficulties. Since
that time (however, the two towns, recognizing the historic interest of
the industry and their commercial dependence on it, have taken steps to
foster it. Schools to teach gemology and the lapidary arts. have been
established to train new workers. Many craftsmen, as a means of
bolstering the economic structured of the community, are now applying
their talents to diamond cutting and the shaping of agate and synthetics
into balance stones, mortars and pestles, bearings and many other kinds
of scientific and technical apparatus. As a result of these and other
constructive
efforts , Idar-Oberstein is now more prosperous than ever before.

Physical

Chemical Composition

Silicon dioxide, or silica,
expressed by the formula SiO2.
The same composition as crystalline quartz, except that it usually contains larger amounts of impurities and may contain some amorphous silica in the form of opal.

Since chalcedony is a crystalline aggregate its optical characteristics cannot be determined by the usual gem testing methods. In the polariscope, translucent stones remain light during rotation.

Pleochroism

None

Dispersion

None

Phenomena

Iris agate exhibits a display of rainbow colors (usually red, green
and blue) caused by diffraction of light from a multitude of very fine growth lines or bands. The so-called fire agate exhibits rainbow colors as a result of
interference of light from exceedingly thin layers of iron oxide
on the deeper layers of the stone.
Both of these are rare.

X-Ray, Fluorescence

All varieties are generally inert under both long and short wavelengths. Some, however, may fluoresce a weak
Green or blue, especially uranium bearing materials from Montana
and Australian chrysoprase.

Chalcedonic quartz, in one form or another, can be confused with
almost every translucent to opaque gemstone. Nephrite, jadeite,
malachite, lapis Lazuli, moonstone, amazonite, turquoise, shell and many
other materials are closely duplicated in appearance by one or more
varieties of natural or dyed chalcedony.

Green chrysoprase and green-dyed chalcedony can be separated from
amazonite by the appearance of minute fractures along the girdle edge.
Amazonite displays cleavage, whereas chalcedony shows only its
characteristic dull to waxy conchoidal fracture. In addition, amazonite
has a characteristics appearance; a sort of shiny reflection that
appears to be just beneath the surface, accompanied by a grid like color
distribution. Chalcedony, on the other hand, is usually rather evenly
colored and lacks both of the characteristics mentioned for amazonite.
They can also be distinguished by a careful spot reading on the
refractometer, for amazonite has slightly lower indices. A heavy liquid
set at 2.57 or 2.58 also separates the two stones.

Chrysocolla chalcedony, which in its finest quality bears a close
resemblance to high-quality turquoise, can be distinguished there from
by physical properties. Chrysocolla chalcedony is considerably less
dense (2.60 to 2.75 plus) and has a lower R.I. (1.535 compared to 1.61).
In appearance they are almost identical, sharing the translucency
expected of the finest turquoise. It is impossible to distinguish
between these two minerals on the basis of fracture or fracture luster,
since both have a conchoidal fracture and a dull to waxy luster on
fracture surfaces.

The mineral chrysocolla, which is distinct from chrysocolla
chalcedony, has a blue enamel like appearance, in a color similar to
that of turquoise, but it is soft (2-4) and crumbles easily. The
refractive indices have been reported from about 1.4 to nearly 1.6, but
1.464 to 1.57 is usual. The S.G. is 2.2 to 2.4. Really, its only
function in the gem field is to act as a coloring agent for chalcedony.

Dyed jasper made to imitate lapis lazuli is seldom a highly effective
imitation on the close inspection. True lapis usually contains flecks of
pyrite, whereas the dyed jasper does not. In addition, the jasper will
give a characteristic chalcedony R.I. of approximately 1.535, as opposed
to approximately 1.50 for lapis.

Jadeite and nephrite are distinguished from chalcedony readily by
either refractive index or specific gravity test, since both jades have
much higher property values.

Although dyed green agate may resemble malachite superficially, the
very high S.G. and R.I. of malachite serve to separate it readily. To
one who is familiar with the appearance of both materials, there is
little resemblance. Malachite often displays a radial fibrous structure
that is never obvious in agate. The two colors in malachite are very
dark green and a lighter green, whereas one color in agate is usually
white or almost white.

One of the key characteristics of chalcedony in its translucent to
semi translucent forms is its behavior in the polariscope; since it is
composed of a multitude of minute crystals with a random orientation, it
remains tight in all positions in this instrument, rather than changing
from light to dark. Moonstone, for example, shows extinction upon
rotation, and in some positions exhibits an interference figure. Another
characteristic of some translucent chalcedony is the fact that in
certain lights it is possible to see a rather shadowy structure, which
would suggest that the material is composed of rounded "blobs" of a
fraction of a millimeter to about one centimeter is size.

Most stained chalcedony has a very intense color that is not
characteristic of the natural. With the exception of chrysocolla quartz,
the color intensity of unstained material is rather subdued. Moreover,
the color of such attractive atones as chrysoprase is usually some what
mottled, in contrast to the solid, even green of the more intensely
colored stained green chalcedony.

Chalcedony is often imitated in glass. Usually, a separation can be
effected by observing the luster of fracture surfaces; that of glass is
vitreous, as opposed to the dull to waxy luster of chalcedony. In
addition, semi transparent to translucent glass often shows bubbles and
swirl marks close to the surface. The dendritic patterns in glass
imitations of moss agate are much too uniform for this substitute to be
confused with the natural stone.

Shell cameos are sometimes confused with stone cameos, especially
those carved from carnelian and white-layered agate or onyx. These may
be distinguished readily by applying a tiny drop of hydrochloric acid,
for the shell effervesces strongly and chalcedony fails to react.
Shell's low (3-4) hardness is also easily detected.

Opal without play of color may closely resemble chalcedony of
comparable color. A spot R.I. reading shows about 1.45 for opal and 1.53
for chalcedony. Translucent opal may well remain light in all positions
in the polariscope; therefore, that instrument does not assist in this
separation, unless the stone remains dark. S.G. differs widely, also.

The chalcedony group of gem varieties offers the consumer an
opportunity to "have his cake and eat it too". These stones, although
lovely, are so plentiful that many are priced at a figure less than
synthetic substitutes for other gemstones. In chalcedony, we see an
excellent example of missed opportunities for jewelry sales. One of
America's fastest growing hobbies is collecting gem materials suitable
for cabochon cutting. The great demand for chalcedony for such purposes
is proof of the beauty that people see in it. It is so inexpensive when
cut abroad that retailer could obtain a large number of stones at a very
low price to feature in window displays and call attention to its
attractiveness for use in cufflinks, tie clasps and also many items of
inexpensive jewelry for women.

Because of the great quantities of chalcedony that are available, the
jeweler should be particular in choosing specimens for stock. It is
frequently possible to purchase good quality for just a few cents more
than poor quality. Since the finer grades are much more attractive and
thus more saleable, it is wise to spend the additional few cents to
obtain stones that will "move".